Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input driven by an engine crankshaft, often via a launch device such as a torque converter, and an output driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns. In front wheel drive vehicles with transverse mounted engines, the engine crankshaft axis is typically offset from the axle axis.
Hybrid electric transmissions further reduce fuel consumption by including one or more reversible electric machines and some type of electrical energy storage such as a battery. Hybrid electric transmissions improve fuel efficiency is several ways. Most internal combustion engines are most efficient when operated at relatively high power settings. A hybrid electric transmission permits operating the engine part of the time at a higher power level than needed for propulsion while storing the excess power in the battery. Then, at other times, the engine is shut off and the vehicle is propelled using the stored energy. Although the engine generates the same amount of total energy, it operates at a higher average efficiency. Also, when the brakes are applied, the reversible electrical machine may capture the vehicle kinetic energy and store it in the battery for later use for propulsion. When the vehicle is stationary and therefore does not require propulsion, the engine can be shut off to eliminate the fuel that would otherwise be used to maintain an idle speed. The electric motor provides the ability to propel the vehicle with the engine off and to rapidly restart the engine when necessary.